Page 37 - Volume 3, Issue 2, July 2010

GC multiresidue method of multiclass pesticides

67

in in solvent isooctane – toluole (90:10) and

in tomato matrix. In tomato matrix, the pres-

ence of two peaks (R.T.=13.7 and 14.9) is ob-

served; the peak with R.T.=13.7 is that of

metribuzin and the peak at R.T.=14.9 is es-

timated to be a metribuzin metabolite. As

shown in Figure 4(b), both peaks have the

same spectra. Therefore, all the steps of the

analytical procedure and the final determi-

nation should be conducted on the same

day to avoid conversion of metribuzin into

its metabolites in the extracts (8, 9). Some-

times, due to the unavailability of the instru-

ment or the large number of required injec-

tions, the recovery experiments may take

24-48 h to be completed and therefore, es-

pecially for metribuzin, we might have con-

version to its metabolites.

The molecular formula of metribuzin is

C

8

H

14

N

4

OS (M.W.=214 for MS purposes) with

an estimated Kovats RI of 1867 i.u., while the

molecular formula of desamino-metribuz-

in is C

8

H

13

N

3

OS (M.W.=199) with an estimat-

ed Kovats RI of 1779 i.u. (11) Therefore, in the

case of these metabolites, the expected re-

tention time (R.T.) should be earlier than the

Figure 4. (a)

Chromatograph of metribuzin at 0.1 mg/ml in solvent isoctane-toluole (90:10) and in tomato matrix.

(b)

Spec-

tra of metribuzin at 0.1 mg/ml in tomato matrix. The upper spectrum is that of an unknown compound and the lower that

of metribuzin.